The present invention relates generally to integrated circuit fabrication equipment. More particularly, the present invention relates to integrated circuit fabrication system for and a method of correcting pupil errors. Even further, the present invention relates to a mask that includes a layer for pupil filtering in a photolithographic camera or stepper unit.
The semiconductor or IC industry desires to manufacture integrated circuits (ICs) with higher and higher densities of devices on a smaller chip area to achieve greater functionality and to reduce manufacturing costs. This desire for large scale integration has led to a continued shrinking of circuit dimensions and device features.
The ability to reduce the size of structures, such as, gate lengths in field-effect transistors and the width of conductive lines, is driven by lithographic performance. In conventional commercial fabrication processes, lithographic systems, such as, photolithographic cameras or stepper units, expose a photoresist material to a pattern of radiation. The photoresist material is developed in accordance with the pattern of radiation to form a pattern of the photoresist material on a wafer. The wafer is processed in accordance with the pattern of photoresist material.
A conventional lithographic system or photolithographic machine can be a projection printing machine using refractive optics in a step-and-repeat projection method. Lithographic systems are sometimes called xe2x80x9csteppersxe2x80x9d, which provide higher image resolution than other scanner-type aligners.
Conventional lithographic systems generally include a light source configured to provide radiation or light at one or more wavelengths. For example, the light source may include an excimer laser producing radiation at a wavelength of 248 nm, 193 nm, and/or 157 nm. The excimer laser can use a KrF source, a ArF source, a F2 source, etc. The lithographic systems can further include a first lens assembly, a mask, and a second lens assembly. The radiation is provided from the light source through the first lens assembly, through the mask, through the second lens assembly to a semiconductor wafer having a layer of photoresist material.
The first lens assembly can be a condenser lens, and the second assembly can be an objective lens. The radiation can be light, such as ultraviolet light, vacuum ultraviolet (VUV) light, and deep ultraviolet (DUV) light. In alternative systems, the radiation can be x-ray radiation, e-beam radiation, extreme ultraviolet (EUV) light, etc.
As described above, conventional lithographic systems can utilize multiple optical elements to focus and direct light to the semiconductor wafer. Generally, the multiple optical elements (e.g., the first and second lens assemblies) can be considered as a single equivalent lens. The pupil of the lithographic system refers to the equivalent lens. The size of the pupil is the diameter of the equivalent lens, and the location of the pupil is the location of the plane of the equivalent lens. The pupil is utilized to mathematically model image formation by the optical elements of the lithographic system.
Lens assemblies of conventional lithographic systems are susceptible to lens aberrations or errors. These errors result in errors in the wavefront that is used by the lithographic stepper unit to produce the image on the wafer. As light passes through the first lens assembly and the second lens assembly, an imperfection in either lens assembly can locally increase or decrease the finite optical path. These imperfections can result in placement errors in the lithographic pattern. These errors are particularly problematic as sizes of lithographic features become smaller.
Accordingly, the pupil of the conventional lithographic system is often tested to determine at which locations errors are introduced into the pupil plane. Heretofore, the pupil of the conventional lithographic systems are probed or tested before installation (e.g., off-line) of the lithographic system by a laser interferometer or by overlay measurement tools. A pupil filter is employed to correct the pupil errors and enhance resolution. The pupil filter is generally inserted within the lens assemblies.
U.S. Pat. No. 6,115,108 discloses a projection-type lithographic system incorporating an illumination modification filter. The illumination modification filter includes a number of distinct illumination filtering zones, wherein each zone is operable to provide a distinct and unique illumination scheme.
A simplified exemplary filter having only three such illumination modification zones is illustrated in U.S. Pat. No. 6,115,108. A first zone of the filter is a pupil filter which inverts or otherwise varies the phase of light which passes through a central, circular region with respect to the phase of the light which passes through an annular region surrounding the inner, circular region. As is well-known by those skilled in the art, such a phase inversion may be provided in either a continuous or step-wise fashion. The pupil filter is constructed by forming a transparent dielectric film over the central region, wherein the thickness of the film may be controlled to provide the desired phase variation.
A second zone of the filter illustrates another exemplary pupil filter, wherein an inner region exhibits a lower transmittance than an outer annular region via a light-absorbing layer formed in the inner region. The transmittance may be modified further by adjusting a radius of the inner region as well as its degree of transmittance. In so doing, a wide variation of illumination schemes may be effectuated therewith. As is well known by those skilled in the art, such a variable transmittance pupil filter may be formed by forming a light-absorbing material, such as a metal film, over the central region, wherein a thickness of the material may be controlled to vary its transmittance.
A third zone illustrates a uniform illumination pupil filter. Such a pupil filter is operable to allow all or some of the illumination light to transmit therethrough in a uniform fashion. Such a filter may be constructed with a uniform transparent plate or alternatively may have a uniform light-absorbing material thereon having a thickness which provides for a uniform illumination attenuation.
U.S. Pat. No. 5,863,712 discloses a pupil filter with variable amplitude transmittance. The pupil filter has a transparent substrate, a phase retarding layer, a translucent film, and an opaque stencil. The pupil filter is replaced or rotated during a number of exposures or during a single exposure. The amplitude of transmittance associated with the pupil filter can be set to prevent imperfect resolution of the lithographic system. However, the variable amplitude transmittance pupil filter is located within the projection lens assembly associated with the lithographic system.
Inserting, adjusting, or rotating pupil filters within lens assemblies is problematic due to the sensitivity of the lens assemblies. In commercial lithographic systems, the lens assembly can be the most sensitive part of the lithographic system. These lens assemblies must be precisely calibrated before commercial operation and are normally serviced by the equipment manufacturer. A pupil filter may even be permanently provided within the lens assembly and may not be adjustable by the IC fabricator. Accordingly, adjusting the lens assemblies to accommodate adjustments to pupil filters is expensive and increases the throughput associated with the lithographic equipment. Further, systems for manipulating the pupil filter within the lens assembly may not be practically available for mass fabrication of ICs.
Pupil filters frequently must be changed for each reticle or mask utilized by the photolithographic system. Each reticle or mask has a different optimum condition which requires a different pupil filter. Accordingly, pupil filtering has been somewhat impracticable for mass fabrication systems which utilize a large number of masks or reticles.
Thus, there is a need for a lithographic system that utilizes pupil filtering to increase resolution for mass production ICs. Further, there is a need for a lithographic system which provides pupil filtering optimized for the mask or reticle utilized by the system. Even further still, there is a need for a process or method of pupil filtering without entering or adjusting components within the lens assemblies. Even further still, there is a need for a reticle or mask that can provide pupil filtering.
An exemplary embodiment relates to a mask or reticle for use in a lithographic system. The mask or reticle includes a substrate. The substrate includes an integrated circuit (IC) pattern and a pupil filter pattern. The IC pattern represents at least one integrated circuit feature.
Another exemplary embodiment relates to a method of enhancing resolution of a lithographic system. The lithographic system includes a pupil. The method includes providing a mask or reticle having an IC pattern and providing a pupil filter pattern on the mask or reticle.
Still another exemplary embodiment relates to a lithographic system. The lithographic system is for use in integrated circuit (IC) fabrication. The lithographic system includes lens assemblies represented by a pupil. The lithographic system includes a mask means for patterning radiation according to an integrated circuit pattern and a pupil filter means for patterning radiation in accordance with a Fourier transform of the shape of the pupil.